Misfires

SMNORMAN

MEMBER
1997 CHEVROLET TAHOE
5.7L
V8
4WD
AUTOMATIC
194,000 MILES

Started off with the lower intake manifold leak. I removed the lower intake with the upper still attached. Set the engine to TDC below I stated. After intake was complete I installed new plugs, wires, rotor and cap.
When I tried to start it, it acted like the old motors (180 off), so I reset and it started fine but the RPM’s went pretty high. I installed a new idle control valve and the RPM’s came down but still a little high (1,500) and there was a misfire. Scanner stated cylinder #1 misfire. I switched #1 and #3 plugs still no change. Switched #1 and #3 wires still the same. All electric connection are okay. Listen for vacuum leak and could not hear one so I sprayed starter fluid without any luck.
So I installed new fuel injectors and scanner states multiple misfires. I also installed new coil and the item mounted next to the coil (forgot the name). Also new MAP sensor.

Have any ideas?

Thanks,
Steve

9 Replies

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Hi,

Have you checked the fuel pressure? Pressure that is too high or low can cause a problem. Also, the engine coolant temp sensor can cause the computer to over or under compensating the air-fuel mixture. I noticed high mileage as well. Low compression can be an issue.

Do me a favor. Start by checking fuel pressure and if you have a live data scan tool, confirm the coolant temperature sensor is sending an accurate signal to the computer.

Here is a link explaining how to test the fuel pressure:

https://www.2carpros.com/articles/how-to-check-fuel-system-pressure-and-regulator

If you don't have a live data scan tool, let me know. Also, let me know if there are any other codes than the P0300 and P0301.

Take care and I'll watch for your reply.

Joe

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Wednesday, May 12th, 2021 AT 8:50 PM

SMNORMAN

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7 POSTS

Thanks Joe for reply. I don’t have a live data scan tool. I have two scanners which just supposedly tell you the codes and can erase “check engine light”.
I checked the fuel pressures as follows.
1. Engine off & ignition on: Design @ 60-66 PSI / Actual 57 PSI
After engine was warm (167 degrees) PSI went up to 59 PSI
2. Engine at an idle: Design @ 55 PSI / Actual 52 PSI and study during the whole test.
3. Engine off temp. @ 106.3 degrees.
4. ECT sensor ohms @ 1.526 (engine off)
5. ECT sensor ohms @. .218 (engine warm)
6. Engine sensor at the left side of the block.
Engine cool ohms @ 1.482
Engine warm ohms @.781
Fuel pressure stayed study during the test.
After engine was turn off for about half hour fuel pressure @ 45 PSI.

Please remember this all started with a rear lower intake gasket leak.
Could the distributor be off a tooth or so?
I have 98 chevy K2500/ 5.7 which runs fine at the moment.
I’m thinking about either just taking the cap and rotor to the Tahoe or set both vehicles to TDC and pull the distributor in the K2500 and set it in the Tahoe.
I’m pulling at strings right now.

Steve

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Saturday, May 15th, 2021 AT 2:38 PM

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Hi,

If it was a timing issue, I don't think it would only affect one of the cylinders. I have to be honest, it sounds like a vacuum issue. Where all did you check. Since it started with the replacement of the intake gasket, are you sure no hoses were cracked, damaged, left off, or anything was left disconnected? Is the misfire code the only code you are getting? Did you check the engine vacuum with a gauge to see if was low which could indicate a leak?

Take a look through this link. It explains how to determine if there is a vacuum-related issue.

https://www.2carpros.com/articles/how-to-use-an-engine-vacuum-gauge

Let me know.

Joe

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Saturday, May 15th, 2021 AT 8:52 PM

SMNORMAN

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7 POSTS

Joe,

I have thought from the beginning there were two items that were causing these problems. One, a vacuum leak and two the distributor. I’ve had other people listen and no one could hear a vacuum leak.
So today I tried again to hear anything and also used my stethoscope and didn’t hear anything. I then went through all electric connection again and made sure there was no corrosion and all connections were good. Well what do you think? The misfire was basically gone but the engine still had this surging (RPM’s jumping up and down 4 to 6 hundred RPMs). Took it for a drive and the misfire was gone but the vehicle would accelerate on its own. I mean it would go as much as fifty MPH on its own. What
I had to put it in neutral at all stops just to keep the surging only in the engine not the whole vehicle.
So may next test was to find that vacuum leak. I did a smoke test through the throttle body and found it leak in only one spot. The injector connector. I also noticed that the connector would not go down completely so the side taps would make.
I stopped at that point because i’m wondering if I should take the whole upper intake back off or should I try and pull the injector body up with something like vise grips.
When I installed this I put lube on the rubber seal around the body and thought the upper intake was pushed down enough but I was wrong.
So what do you think?

Thanks,

Steve

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Sunday, May 16th, 2021 AT 6:42 PM

JACOBANDNICKOLAS

MECHANIC
108,192 POSTS

Hi,

When you say injector connector, I'm not sure what you are referring to. No smoke should come from electrical connections. Do you mean it's coming from around the o-rings on the injector?

If possible, take a pic where the leak is and upload it for me to see. Also, let me know if it involves cylinder one. One more thing, it can be difficult to hear a vacuum leak when the engine is running. Did you try using carburetor cleaner?

Let me know.

Joe